Hydrofluorocarbons (HFCs) and in particular hydrofluoro-olefins (HFOs), such as 2,3,3,3-tetrafluoro-1-propene (HFO 1234yf) are compounds that are known for their properties as coolants and heat-exchange fluids, extinguishing agents, propellants, foaming agents, swelling agents, gaseous dielectrics, polymerization medium or monomer, support fluids, agents for abrasives, drying agents and fluids for energy production units. Unlike CFCs and HCFCs, which are potentially hazardous to the ozone layer, HFOs do not contain chlorine and thus pose no problem to the ozone layer.
HFO 1234yf may be obtained by dehydrofluorination of a pentafluoropropane. The reagent concerned may be 1,1,1,2,3-pentafluoropropane (HFC 245eb), as described in document WO 2008/002 500, or 1,1,1,2,2-pentafluoro-propane (HFC 245cb), as described in document WO 2007/079 435. These pentafluoropropanes are prepared by fluorination in the liquid or gaseous phase of chloro or chlorofluoro compounds or via successive dehydrohalogenation and hydrogenation reactions. Document U.S. Pat. No. 5,396,000 describes, for example, the preparation of 1,1,1,2,3-pentafluoropropane (HFC 245eb) via catalytic dehydrohalogenation of 1,1,1,2,3,3-hexa-fluoropropane (HFC 236ea) into 1,2,3,3,3-pentafluoro-1-propene (HFO 1225ye), followed by a hydrogenation to produce the desired compound.
HFO 1234yf may more generally be obtained by dehydrohalogenation of a tetrafluorohalopropane. The reagent concerned may be 1,1,1,2-tetrafluoro-3-iodopropane, as described in document WO 2005/108 334, or 1,1,1,2-tetrafluoro-2-chloropropane (HCFC 244bb), as described in documents WO 2007/079 431 and WO 2008/040 969.
All these processes start from or proceed via saturated halofluoro compounds. On account of the large differences in volatility between these saturated halofluoro compounds and HFO 1234yf, they may generally be separated via standard distillation processes. However, out of concern for obtaining a product of maximum purity, it is appropriate to lower the residual concentration of saturated impurities.
The use of adsorbent solids for the purification of fluoro compounds is already known. The purification treatments are usually performed at room temperature or in the region thereof.
Document JP 2002/226411 describes the purification of 1,1,1,3,3-pentafluoropropane (HFC 245fa) comprising between 5 ppm and 2% by weight of halopropene, such as fluoropropene and chlorofluoropropene, using a solid adsorbent, especially active charcoal.
Document U.S. Pat. No. 7,084,315 describes the removal of olefinic impurities in a hydrofluoroalkane, typically the removal of 1-chloro-2,2-difluoroethylene (F1122) in 1,1,1,2-tetrafluoroethane (F134a) over molecular sieves. HFO 1234yf may be present as impurity.
Document U.S. Pat. No. 7,041,264 describes a process for purifying octafluoropropane, comprising a step of placing crude octafluoropropane containing impurities in contact at high temperature with an impurity-decomposing agent, followed by a step of removing these impurities, for example using an adsorbent. The preferred decomposing agent comprises an iron oxide and an alkaline-earth metal compound.
Document U.S. Pat. No. 7,094,935 describes a method for purifying octafluoropropane or octafluorocyclobutane using an adsorbent obtained according to a process comprising (i) a step of acidic washing of charcoal, followed by washing with water, (ii) a step of deoxidation and/or dehydration of the charcoal, (iii) a recarbonization step at between 500 and 700° C. and an activation step at a temperature of between 700 and 900° C. under a gaseous stream comprising an inert gas, carbon dioxide and water vapor.
Document WO 2007/144 632 describes the use of molecular sieves, with pore sizes of between 3 and 5 Å, for controlling the humidity level of a refrigerant fluid comprising a fluoropropene, especially HFO 1234yf, optionally mixed with iodotrifluoromethane and/or a lubricant.
Document JP 1040507 teaches the use of molecular sieves for reducing the content of perfluoro-2-butyne, an impurity present in hexafluoropropene, to less than 5 ppm.
In patent WO 08/001 844, crude hexafluoropropene is placed in contact with an adsorbent comprising a zeolite whose micropores have a mean diameter of between 3.4 and 11 Å and/or a charcoal-based adsorbent whose micropores have a mean diameter of between 3.5 and 11 Å, for reducing the content of chloro compounds and/or of hydrocarbons in the hexafluoropropene.